Featured Species Articles

Why does mimicry evolve?

Mimicry refers to shared warning coloration between co-occurring species. There are two main classes of mimicry: Batesian, and Müllerian. Batesian mimicry is when a non-toxic species resembles a toxic species. The benefits of Batesian mimicry are fairly obvious: by resembling a toxic species, a non-toxic species “tricks” a predator into thinking it is toxic, and thus avoids being attacked. Müllerian mimicry is when two (or more) toxic species share a common warning coloration. This spreads the cost of predator learning across a larger pool of individuals, reducing the per-capita mortality rate. In Müllerian mimicry, because both species are toxic and appear similar, a predator attacking species A will then avoid species B, and vise-versa. It is therefore a mutualistic relationship.

Slideshow 1 – Basics of Müllerian mimicry

An important point about Müllerian mimicry (and aposematic signals in general) is that they are positive-frequency dependent. This means they have “strength in numbers”: the more abundant a warning signal is, the greater protection it confers to those who possess it. This explains the elaborate mimicry rings in tropical butterflies, with numerous species “participating” by converging on a single wing pattern. This positive-frequency dependence also means that novel phenotypes that do not conform to the local mimicry ring will be at a disadvantage. Because these individuals are not recognized by predators as being toxic, they will be selectively attacked. This means that once a mimetic morph is established in an area, it will be stable and resistant to invasion by a foreign morph.

Mimicry in Ranitomeya imitator

In north-central Peru, the dendrobatid species Ranitomeya imitator bears a striking resemblance to other species of Ranitomeya (Figure 1).

Figure 1 – Mimicry in Ranitomeya imitator

These morphs occur in different geographical areas, forming a “mosaic” of mimetic morphs across the landscape (Figure 2).

Figure 2 – Distribution of mimetic morphs of R. imitator

The possibility of Müllerian mimicry in R. imitator was recognized by Rainer Schulte in the original description of the species (Schulte 1986). However, it was not rigorously tested till much later.

A key aspect of mimicry is that warning coloration is shared not by common ancestry, but by convergent evolution. In other words, if two species appear similar because they are very closely related, this is not mimicry but simply a symplesiomorphy (shared ancestral character). Symula et al. (2001) provided phylogenetic evidence that that color similarity between R. imitator and its co-mimics was not due to common ancestry, thus supporting the hypothesis of Müllerian mimicry. They found that all populations of R. imitator—despite their highly variable coloration—were members of a single, variable species, and that the co-mimic species were more distantly related. This meant that the color morphs found in R. imitator evolved after the split from its co-mimics, supporting the mimicry hypothesis.

Another important aspect of Müllerian mimicry is that co-mimics are (a) both toxic and (b) confer mutual protection on one another. In other words, a predator attacking species A will learn to avoid species B, and vise-versa. Surprisingly, these predictions were not tested in R. imitator until over a decade later, when Adam Stuckert published two papers on this topic. Alkaloid data confirmed that R. imitator was indeed toxic, possessing quantities of alkaloids comparable to (if not exceeding) that of the co-mimic species (Stuckert et al. 2014a). A predator-learning experiment further found that predators attacking the spotted morph of R. imitator learned to avoid attacking its co-mimic, the spotted morph of R. variabilis, and vise-versa (Stuckert et al. 2014b).

Mimicry as a cause of population divergence

While the evolution of mimicry causes phenotypic convergence among unrelated species, it can also drive phenotypic divergence within a species. If a species co-occurs with different potential model species, different mimetic morphs can be established in different geographical areas. Because these morphs are under local positive frequency-dependent selection, they are resistant to invasion by foreign morphs. This can lead to the formation of narrow transition zones between different mimetic morphs. In R. imitator, three such zones have been identified (Twomey et al. 2014, 2016).

Slideshow 2 – The establishment and spread of mimetic morphs and the formation of transition zones.

Mimetic transition zones are of considerable interest for evolutionary biologists as they yield insights as to how populations of a single species diverge and become reproductively isolated, which may give clues about how new species are formed. For example, transition zones that are very narrow indicate that there is strong selection against immigrant phenotypes crossing the zone (crossing either physically by dispersal, or “crossing” indirectly via morph hybridization). Morph-based assortative mating may also reinforce morph boundaries, leading to narrow transition zones. Ultimately, strong selection and assortative mating may reduce gene flow between morphs, leading to genetic structuring that coincides with the mimetic transition zone.

Slideshow 3 – Mimicry “transition zones” in Ranitomeya imitator.

In R. imitator, field work has led to the discovery of three mimetic transition zones: striped-banded, striped-spotted, and striped-varadero. These transition zones are characterized by narrow areas of rapid phenotypic change. For example, in the striped-banded transition zone, the Huallaga Canyon provides a “natural” transect: in the upper and lower areas of the canyon, pure banded and striped morphs are found, respectively. In the central areas, there is a shift from banded to striped phenotypes that occurs over a relatively short stretch of river (~5 km). In addition to the rapid color pattern shift, the central populations are much more phenotypically variable than the “parent” populations, consistent with the idea that these populations are mimetic hybrids.

In the case of the striped-banded transition zone, there is a shift in color pattern elements, specifically the dorsal coloration, dorsal pattern, and leg color/pattern. However, there is no evidence of assortative mating at the transition zone, and no evidence of a reduction in gene flow between morphs (Twomey et al. 2016).

The striped-spotted transition zone is similar in the sense that there is a shift in color pattern elements, but no evidence of reduced gene flow. This transition zone occurs at the lowland/highland transition north of Tarapoto, and tracks the variation seen in the polymorphic R. variabilis.

The striped-varadero transition zone is different in a few ways. First, it is much narrower than the other zones, approximately 2 km wide. Second, there is some evidence for assortative mating at the morph boundary, with the local striped morphs showing a preference for their own morph (Twomey et al. 2014). Third, there is relatively strong genetic structuring at the transition zone, indicating that the two morphs may be reproductively isolated. Finally, the two morphs are different in aspects seemingly unrelated to mimicry (but possibly related to reproduction): size (with the varadero morph being much larger), and calls (with the varadero morph having a lower, longer call). Because of the narrowness of the zone, the presence of assortative mating, and the reduction in gene flow, this zone may be indicative of an early, but incomplete stage of speciation that is being driven by Müllerian mimicry.

FOR SALE: Rarest Newest Species

It seems to be an inherently human characteristic to desire that which we do not have. Acquisition of a new species is one of the most exciting parts of this hobby. Every hobbyist has his or her own wish list, some of which are easily obtainable, others far more elusive. How these wish lists are filled can speak volumes about the integrity of the hobby and the individual hobbyists. The desire for new and rare species has directly fueled the exploitation of these frogs in the wild through a destructive practice known as smuggling.

What is smuggling?

With many amphibian species disappearing worldwide due to disease, global climate change, and deforestation, many conservation efforts have been spawned lately to investigate and combat such threats. However, poison frogs face a threat that has largely been overlooked – illegal smuggling for the pet trade. The last several years have witnessed recent waves of large volumes of smuggled frogs appearing at frog shows or expos, particularly in Europe, however N. America is far from innocent. Some species of particular note are Dendrobates mysteriosus, D. granuliferus, D. fantasticus, D. imitator, D. vanzolinii, many forms of D. lamasi (most recently the orange/red form), and D. lehmanni. The list is extensive, and continues to grow.

Smugglers exploit the often under-funded governing bodies and enforcement agencies in the countries in which these frogs are native. In Peru, smugglers either carry the frogs out of the country personally, usually several hundred at a time, hidden amongst luggage, or export the frogs illegally, hidden within shipments of tropical fish leaving through Iquitos or Lima. Many times large quantities of frogs are harvested from often very restricted populations, in most cases severely damaging these populations. Furthermore, impoverished Peruvian farmers are often paid ridiculously low sums of money ($1-2) to collect every poison frog they can find. Not only is this damaging to the frog populations, it instills in these farmers the idea that the forest and its inhabitants are just another resource to be carelessly exploited rather than conserved or managed sustainably.

These frogs, whether concealed in luggage or hidden with fish, are horribly and inhumanely packed, often resulting in the death of 100% of these illegal shipments. Seizure of illegal frogs in Iquitos revealed several film canisters packed with D. lamasi, often literally packed solid with frogs. The frogs face death as a result of the deplorable shipping conditions, or as the result of injuries received during the stress of such conditions, or from bacterial, fungal or viral infections which take over a weakened, stressed immune system. For every smuggled frog that arrives alive, there are many, many more that die. Customs officials have quoted mortality to be as high as 90%. Similar numbers have been observed within Peruvian seizures; sadly, one must be reminded that many more died before they could even be packed for transport.

Two months ago a new variant of D. imitator arrived Europe, recently we had the opportunity to return the area the frogs were from and see the impact. The site was dismal. Because the smugglers created a market for these frogs, half a dozen campensinos (farmers) had begun collecting the frogs and placing them in small enclosures. The mortality of the frogs waiting for export had to be considerably high as all the frogs we observed were severely starved and seemed to be barely alive. Further, a considerable amount of frogs which didn’t starve to death could have easily died of desiccation, excessive heat, predation, or disease, as many of the cages were so poorly built frogs could easily escape. However, if they were able to escape they were now miles away from suitable forests and surely died. Since the smugglers are stealing these frogs, to them, these losses are acceptable and they simply remove more from the wild. They will try again and again when mortality is high. To the campensinos, most of these frogs can be collected while they work in the forests, so the small chance that they live to be exported is still worth the effort. Smuggling has unfortunately become a highly lucrative venture benefiting only the smugglers themselves, as the short term income to the farmer hardly ameliorates their poverty.

How is it harmful?
Those who knowingly purchase illegal frogs are every bit as guilty as the smugglers themselves and by doing so are directly encouraging this trend to continue. There is no justification for these practices. No one wins, not the frogs, not conservation, not the hobby, and certainly not the local communities. Supporting these smugglers is directly supporting the extirpation of these frogs from the wild, and making it much more difficult for the sustainable projects to succeed.

No one can deny the clandestine origins of many of the now common frogs in the hobby, whose origins were smuggled animals. However, our hobby does not have to continue this way. Never in the history of the poison frog hobby have greater opportunities been available to the hobbyist to purchase a wide variety of sustainably produced frogs , which come with the additional benefit that portions of proceeds go to protect the very habitat these frogs come from. In many cases, supporting these projects aids the economies of impoverished local communities by providing them a sustainable source of income and employment opportunities derived from the forest, rather than from unsustainable exploitation.

Smuggling directly undercuts these programs. Projects such as INIBICO and Zoocriadero Exotics Frogs (UE) will not thrive if the market for these frogs is constantly filled by smuggled animals. These projects have significant overheads, operating costs, and support several employees. They will simply cease to function if they cannot make the sales they need, and disappearing with these projects would be the conservation potential these frogs have to offer.

What can be done?
Unfortunately, many hobbyists choose to ignore the possibility that frogs they are purchasing may be smuggled, or simply take on an apathetic “don’t ask, don’t tell” mentality, naively assuming that the frogs they are purchasing are of legal origin. We urge hobbyists to ask questions when buying any frog newly arriving on the market. It is a simple matter of asking the seller questions: Where are these frogs from? How were these frogs obtained? If they are captive bred, how were the parents obtained? Could you provide CITES documentation? Any legal seller should have quick answers to these questions and will be able to provide documentation for the frogs they are selling.